full time primary education - what does it mean?

we are having difficulty with our DS - getting him to go full-time at primary school. He is currently attending half day - but we are obviously concerned that he is missing out. Things are slightly complicated as the school has recently converted to an academy. He does not have a statement but has been on early years action, and action plus. School say they do not have funds available to pay for additional teaching assistant?

Does anyone know the exact statutory requirement for what they must provide please? (links to official documents or policies would be great) We would like to be able to challenge them with their duty vs what they currently offer.

Heard / read different things about this - would be great to get something definitive.